With the diversified advance of the electronic technology, there are diversified performance requirements of concern for insulating materials intended for use in electronic devices. In particular, printed wiring boards have been used in a very wide range of application, and the performance requirements on substrates thereof have been diversified. Under the circumstances, there are many different requirements regarding dielectric characteristics.
Low-dielectric constant printed wiring boards have been developed with a focus placed on high speed propagation, high characteristic impedance, size reduction or cross-talk reduction. On the other hand, high-dielectric constant substrates are needed to meet such demands as the formation of delay circuits in high frequency and microwave circuits and other wiring boards, matching of the characteristic impedance of wiring boards in low impedance circuits, miniaturization of wiring patterns, and fabrication of hybrid circuit devices on substrates having a capacitive effect.
As the information communication system advances, mobile communication and satellite communication systems such as automotive radiotelephones and digital mobile phones now become of great interest, which use a high frequency band of the order of megahertz to gigahertz as the frequency band of radio wave. While communication instruments used in these communication means are in rapid progress, attempts have been made for the size reduction and high density packaging of casings and electronic parts. Similar requirements are imposed on the antennas used in the communication instruments. Planar antennas having micro-strip lines formed on dielectric substrates are used as high-frequency antennas.
The dielectric substrates used in these applications should have a high dielectric constant and a low loss so that the planar antennas can be reduced in size.
Electronic parts such as strip lines, impedance matching circuits, delay circuits and antennas should have high Q values because satisfactory characteristics are not available with low Q values. On the other hand, in the fabrication of resonators using strip lines, a high dielectric constant is necessary for size reduction purposes because the length of strip lines is in proportion to 1/√∈ wherein ∈ is a dielectric constant.
Capacitors having an increased capacitance are generally produced by spreading the area of opposed electrodes, increasing the number of layers, or reducing the distance between layers. These approaches increase the size or thickness of substrates or parts.
For such capacitors, a high dielectric constant is necessary for size reduction purposes.
As one suitable dielectric material, JP-A 9-31006 discloses a polyvinylbenzyl ether compound of a specific structure in the cured state. This compound fails to achieve a high dielectric constant in the high-frequency band. There is a need to have a material satisfying such properties.
In the prior art relating to such applications, high dielectric constant substrates are proposed which are obtained by stacking and molding prepreg sheets which are, in turn, obtained by adding a ceramic powder having a high dielectric constant to an epoxy resin (for laminates or printed circuit boards) or a polyphenylene ether resin (which is a low dielectric constant resin), impregnating glass fabric or glass non-woven fabric therewith, followed by drying.
However, the dielectric dissipation factor cannot be reduced merely by adding a high-frequency ceramic powder having a high dielectric constant to conventional thermosetting resins as typified by epoxy resins for prior art laminates or printed circuit boards. Where high dielectric constant fillers are added to polyphenylene ether resins which are low dielectric constant resins, the dielectric dissipation factor becomes low. However, the amount of the filler added must be increased in order to provide a high dielectric constant, which gives rise to problems including difficulty to drill and machine the laminate and substantial dimensional variances.
JP-A 9-31006 discloses a resin obtained by polymerizing or curing a polyvinylbenzyl ether compound of a specific structure, the resin exhibiting dielectric characteristics which are satisfactory and constant over a wide frequency region and least dependent on temperature and moisture absorption as well as heat resistance. Since this resin has a low dielectric constant and a low dissipation factor, it fails to fulfil the requirement in some applications where a high dielectric constant is needed.
The substrates used in the fabrication of electronic parts and circuit boards include composite substrates which are obtained by mixing a molding material with ferrite powder, molding the mixture into plates, and treating the plates as by electroplating, for example, composite ferrite substrates molded from composite ferrite substrates materials comprising a liquid crystal polymer and ferrite. Also included are copper-clad laminates using prepreg sheets formed from ferrite powder-free, glass cloth-reinforced epoxy resins or phenolic resins.
However, in the case of the molded plates treated as by plating, it is difficult to mold thin-wall plates of large planar dimensions. The copper-clad laminates which are free from ferrite powder, that is, lack magnetic material have the problem that in forming devices, parts and circuits utilizing magnetic characteristics, a ferrite material must be separately applied or a ferrite member must be mounted. The copper-clad laminates which are free from ferrite powder do not have magnetic shielding effects by themselves and are not suitable in magnetic shielding purposes.
JP-A 58-158813 discloses an electrical laminate comprising a base impregnated with a laminate-forming resin containing a metal oxide having both magnetic and electrically insulating properties. Illustrative examples are combinations of phenolic resin with kraft paper, which are poor in heat resistance and strength required for thinning purposes. The content of ferrite powder is below 50 wt % of the entire composition, failing to provide satisfactory magnetic properties required as a magnetic material.
JP-A 59-176035 discloses a composite fiber material for absorbing electromagnetic waves, comprising fiber layers disposed one on top of the other and joined by a matrix consisting of a resin and a curing agent wherein a filler for absorbing electromagnetic waves is contained in each layer such that its concentration is graded from the outside to the inside. Since the filler is distributed so as to give a compositional grading, the prepreg manufacture is cumbersome.
JP-A 2-120040 discloses a copper-clad laminate for absorbing electromagnetic waves, which is obtained by impregnating glass fiber woven fabric with a thermosetting resin, drying to form a prepreg, and placing copper foil on the prepreg, followed by laminating press, wherein an electromagnetic wave-absorbing material is mixed and dispersed in the thermosetting resin so that electromagnetic noise of a selected frequency is absorbed. Since PZT powder is used in illustrative examples, the resulting laminates are not suitable in magnetic property-utilizing applications and magnetic shielding purposes.
JP-A 11-192620 discloses a prepreg obtained by kneading ferrite powder and an epoxy resin in a solvent to form a slurry paste, and impregnating glass cloth with the paste, followed by drying, and a composite magnetic substrate obtained by laminating press of the prepreg. Since the epoxy resin used as the base of the prepreg has a high dielectric constant, the resulting composite magnetic substrate naturally has a high dielectric constant and high dissipation factor. Because of a relatively high percent water absorption, the substrate is likely to undergo a pattern peeling phenomenon and changes of dielectric constant and dissipation factor during solder flow and dipping steps.
JP-A 10-79593 discloses a prepreg obtained by impregnating glass cloth with a magnetic paint comprising a soft magnetic powder and a thermosetting resin, and a printed wiring board. Since an epoxy resin used as the base of the prepreg has a high dielectric constant, the resulting composite magnetic substrate naturally has a high dielectric constant and high dissipation factor. Because of a relatively high percent water absorption, the substrate is likely to undergo a pattern peeling phenomenon and changes of dielectric constant and dissipation factor during solder flow and dipping steps.
Polyvinylbenzyl ether compounds are combustible and so, safety becomes a problem when they are applied to multilayer substrates and electronic parts. It remains unsolved to manufacture multilayer substrates and electronic parts that clear UL-94, V-0 rating.
JP-A 9-31006 discloses a polyvinylbenzyl ether compound and a method for preparing the same. This polyvinylbenzyl ether compound in the cured state has dielectric characteristics which are satisfactory and constant over a wide frequency region and least dependent on temperature and moisture absorption, as well as good heat resistance.
It is described in JP-A 9-31006 that the polyvinylbenzyl ether compound is prepared by reacting a polyphenol with a vinylbenzyl halide in a polar neutral solvent in the presence of an alkali metal hydroxide as a dehydrochlorination agent, or in a water/organic solvent mixture in the presence of a phase transfer catalyst (e.g., quaternary ammonium salt) and an alkali metal hydroxide as a dehydrochlorination agent at a temperature of up to 100° C. The polyvinylbenzyl ether compound thus obtained is directly polymerized or cured into a cured product. The cured product of polyvinylbenzyl ether compound obtained by this procedure, however, does not have the desired dissipation factor and are not suitable for use in the high-frequency application. The transmission loss of a signal is represented by the product of frequency, square root of dielectric constant, and dissipation factor, which means that a lower dissipation factor among dielectric characteristics becomes desirable as the frequency becomes higher.
Commonly known high-frequency electronic parts and multilayer substrates include those obtained by stacking multiple layers of sintered ferrite or sintered ceramics and molding them into the substrate shape. This has been a common practice because the multilayer substrates resulting from these materials have the great advantage of size reduction.
However, since sintered ferrite material has the problem that the frequency response of magnetic permeability μ among magnetic characteristics merely extends up to about 500 MHz, its use in a high-frequency band of the order of gigahertz is limited. The material has a large dielectric constant and suffers from a lowering of high-frequency characteristics under the influence of stray capacity.
Besides, simply using sintered ceramics encounters difficulty in achieving a dielectric constant of 4 or less. A further lowering of dielectric constant is desired in order to enhance high-frequency characteristics.
For enhancing high-frequency characteristics, JP-A 9-76341, 11-192620 and 8-69712 disclose substrates of composite materials comprising a ceramic magnetic material such as sintered ferrite or ceramic dielectric material and an organic resin material. Nevertheless, there is yet available no material that meets the desired high-frequency characteristics.
Where heterogeneous materials such as sintered ferrite and sintered ceramic are contained in a common multilayer substrate as multiple layers, there arises the problem that cracks often occur due to the difference of coefficient of linear expansion.